This invention is concerned with a picture display panel for a cathode ray tube employed in a color television receiver, and with an improved glass from which such panel is produced.
A conventional cathode ray tube for color television reception consists basically of a glass display panel, a pattern of phosphor configurations forming a screen on the interior of the panel, a tubular neck portion in which an electron gun is mounted in fixed position, an intermediate funnel portion spacing the gun from the panel, and an electron mask. During tube operation, the mask, which is provided with a pattern of perforations, intercepts in part the electron gun output and transmits the remainder as electron beams. These beams impinge on the phosphor configurations to produce the color picture.
The phosphor configurations, commonly in groups of three, may take various forms, but are usually referred to as dots. It is, of course, critical that the electron passages in the mask, and the resultant electron beams, be carefully aligned with the phosphor dots, and that such alignment is maintained as closely as possible throughout the assembly and processing of the tube.
It was recognized, at least as early as 1964, that alignment of the electron passages in the mask with the phosphor dots could be lost during thermal processing operations, such as frit sealing of the tube components and bake-out of the phosphor screen. The nature and effect of the problem are described in some detail in U.S. Pat. No. 3,357,767, granted Dec. 12, 1967 to P. C. Shaffer.
It has been found that this source of misalignment arises in part at least from a change in glass structure during the thermal treatment, the phenomenon being known as compaction and resulting in a change in glass density. Thus, the ordinary glass annealing process, while adequate to reduce observable strains in the glass, does not actually produce a glass body that is structurally stable. Accordingly, the Shaffer patent proposed to solve the problem by subjecting the glass panel to a preliminary heat treatment equal in degree and in time to the heat treatment that the glass would ultimately be exposed to during tube manufacture.
The problem of glass compaction, and resultant loss of alignment during thermal processing, can also be remedied by prolonging or extending the normal glass annealing schedule to approach the fine anneal that is commonly used in obtaining refractive index control in optical glasses. While such extended and/or separate steps are obviously cumbersome and expensive, they nevertheless are resorted to in color television tube production.